switch vcpkg to manifest mode; fix eigen version

[antond-weta]

Description

Eigen 3 has been updated to 3.4.1 on vcpkg with broken build scripts.
Restricting a package to a specific version is not supported in the command line mode, so I'm switching vcpkg to manifest mode, and adding Eigen 3.4.0 to the manifest.

Tests

no code changes, any changes to Eigen should be covered by the existing tests.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.27%. Comparing base (bbb6200) to head (164602a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #209   +/-   ##
=======================================
  Coverage   72.27%   72.27%           
=======================================
  Files          10       10           
  Lines        2348     2348           
  Branches      311      311           
=======================================
  Hits         1697     1697           
  Misses        651      651           

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